Given a grammar and a token pattern constructor, returns the list of
the corresponding values currently used in all entries of this grammar.
The integer is the number of times this pattern value is used.

Examples:

If the associated lexer uses ("", xxx) to represent a keyword
(what is represented by then simple string xxx in an EXTEND
statement rule), the call Grammar.token g "" returns the keywords
list.

The call Grammar.token g "IDENT" returns the list of all usages
of the pattern "IDENT" in the EXTEND statements.

Alternative for grammars use. Grammars are no more Ocaml values:
there is no type for them. Modules generated preserve the
rule "an entry cannot call an entry of another grammar" by
normal OCaml typing.

Grammar.iter_entry f e applies f to the entry e and
transitively all entries called by e. The order in which
the entries are passed to f is the order they appear in
each entry. Each entry is passed only once.

Grammar.fold_entry f e init computes (f eN .. (f e2 (f e1 init))),
where e1 .. eN are e and transitively all entries called by e.
The order in which the entries are passed to f is the order they
appear in each entry. Each entry is passed only once.